Simultaneously upgrading the power conversion efficiency and the moisture resistance of perovskite solar cells by blending CsI into carbon-electrode

Three kinds of alkaline-metal iodide salts (CsI, LiI, and KI) are blended in the carbon paste, which is then used to coat the carbon-electrode (CE) on perovskite (PVSK) layer, then the resultant CE based perovskite solar cells are treated in a damp chamber with relative humidity of 85%. It is observed that the salt treatment could raise the power conversion efficiency of the devices, and CsI treatment comes out with the fastest rising rate. X-ray diffraction and scanning electron microscope studies observe that CsI treatment triggers secondary growth of PVSK, as well as the doping behavior at the A site of PVSK. Transient photovoltaic/photocurrent decay curve recording and Mott–Schottky study on the devices show that CsI treatment comes out with the longest lifetime of charge carriers, the shortest charge-extraction time, and the highest built-in potential. Accordingly, CsI blending upgrades the power conversion efficiency from 17.22% to 19.16% (on average, optimized to 19.89%), leading to an increment of 11%, compared to that of 7%,1% and 2% for LiI, KI, and control cases, respectively. Moisture stability is estimated. CsI blending comes out with a T80 lifetime (time for efficiency dropping to 80% of the initial value) of ∼209 h, compared to that of ∼180h, ∼67h, and ∼90h for LiI, KI, and control cases, respectively. The observation is ascribed to the improved crystallization, and molar Gibbs free energy of hydration (ΔhydG*) for these alkaline-metal ions. Cs+ owns the lowest ΔhydG*, thus enhancing the hydrophobicity of the CE. Besides, Li+ owns the highest ΔhydG*, thus triggering the “slow-release effect” on H2O molecules from open air.